There is a global urgency to develop a protective vaccine against HIV-1. Neutralizing antibodies (Nabs) can provide effective prophylaxis against HIV-1 infections. However, eliciting Nabs that are broadly cross-reactive against many antigenically diverse HIV-1 isolates has been a major challenge and it remains a critical roadblock to HIV-1 vaccine development. The primary objective of the studies being proposed is to generate novel antigens that are able to elicit such Nabs, with a long-term goal of developing a vaccine against the virus. The underlying hypothesis of this proposal is that we will be able to enhance immunogenicity of key epitopes that can elicit broadly reactive Nabs by altering the antigenic composition of HIV-1 envelope protein, as long as the immunogen is structurally intact and the epitopes are antigenically correct. To test this hypothesis, we have generated prototypic antigens based on gp41 membrane-proximal external region (MPER) and gp120 outer domain (gp120OD). Both proteins are immunogenic and can elicit Nabs against multiple primary HIV-1 isolates. Our goal is to use current and evolving understanding of biochemical, structural and immunogenic properties of the prototypic antigens to design second-generation antigens that could induce even more potent Nabs. The proposed studies are focused and hypothesis-driven, with clearly defined milestones and timelines. Successful completion of the proposed studies would represent a major advancement towards developing a protective AIDS vaccine. The proposed research Program consists of two Projects and one Core: The role of Project 1 is to design subunit envelope antigens, to produce them, and to evaluate their immunogenic properties. The role of Project 2 is to determine high-resolution structures of the antigens with a goal of facilitating the antigen design process and understanding of their immunological properties. The Administrative Core is responsible for providing the organizational management and maintaining infrastructure to support the fiscal monitoring.